Cell populations are often heterogeneous with regard to the ability of their members to respond to stimuli such as hormonal growth factors. As a result, they may achieve a graded response at the level of the whole cell population, even though the response may be all-or-none at the level of the individual cell. the proposed research uses cultured human diploid fibroblasts as an experimentally accessible model for analysis of the epi-genetic mechanisms underlying heterogeneity of proliferative responsiveness. Because this heterogeneity changes throughout the life of the fibroblast population, diploid fibroblasts are also a widely-used model system for the study of cellular aging. The cyclins are a recently-discovered class of proteins that control the ability of cells to divide. We found that the level of cyclin mRNA decreases throughout the life of the fibroblast population. The mRNA could therefore be the physiological 'memory' that determines the number of population doublings before the fibroblasts senesce. If so, heterogeneity of the cells' ability to divide should be accompanied by heterogeneity in the cells' expression of cyclin-like genes. Accordingly, the general objective of this project is to develop biomathematical models that may be used to predict the relation between heterogeneity of the cells' ability to divide (measured by time-lapse video microscopy) and the expression of cyclin-like genes (measured by quantitative nucleic acid hybridization). Specifically, we propose to develop biomathematical models to: (1) analyze the hypothesis that in cell populations that are grown under optimal growth conditions, heterogeneity of the cells' ability to proliferate is due to random autocatalytic synthesis and degradation of cyclin-like mRNA, as well as random partitioning of the mRNA during cell division; (2) analyze the hypothesis that during quiescence, cells maintain a 'memory' or cyclin mRNA levels during prior growth, unless the quiescence is so deep that the population's lifespan is shortened; (3) analyze the hypothesis that chemical agents such as retinoic acid and TPA modulate the lifespan of fibroblast populations by affecting the cell-to- cell heterogeneity of cyclin mRNA levels.